THE IMPACT OF SUBDUCTION-ZONE METAMORPHISM ON MANTLE-OCEAN CHEMICAL CYCLING

Subduction zones represent avenues of recycling of Crustal, atmospheri c and oceanic (including organic) components to the mantle; metamorphi sm in the forearc and subarc regions of subduction zones likely dictat es the extent to which elements are retained in subducted rocks into t he deep mantle or participate in shallower fluid- and melt-related pro cesses (are magmatism, production of forearc serpentinite mud volcanoe s, seafloor trench fluid venting). Knowledge of the impact of subducti on-zone metamorphism on the compositions of subducting sediment and oc eanic lithosphere may yield significant insight into the processes lea ding to are magmatism and mantle chemical heterogeneity. Devolatilizat ion processes in the Catalina Schist subduction-zone metamorphic terra ne (California) resulted in the release of volatiles from metamafic an d metasedimentary rocks and the large-scale transport of H2O-rich C-O- H-S-N fluids with relatively uniform O- and H-isotope compositions. In the metasedimentary rocks of the Catalina Schist, the concentrations of elements such as B, N, Cs, As, Sb and possibly U (and the ratios of the concentrations of these elements to the concentrations of less fl uid-mobile elements such as the HFSE and REE), the B/Be ratios, and th e H2O contents were decreased by progressive devolatilization. The Rb/ Cs and C-reduced/N ratios, delta(15)N, and delta(13)C of the Catalina Schist metasedimentary rocks were increased during the high-P/T metamo rphism. Metabasaltic rocks, despite their metamorphism at up to amphib olite-facies P-T conditions and resultant dramatic loss of H2O, preser ve major and trace element and isotopic compositions similar to those of hydrothermally altered seafloor basalts (e.g., enrichments in B, K, Rb, Ba, and Cs, decreases in K/Rb and K/Cs, elevated delta(18)O and S r-87/Sr-86(i) relative to unaltered MORB). Metamorphism along the fore arc slab-mantle interface may prevent the deep subduction of many subd ucted volatile components (e.g., H2O, Cs, B, N, perhaps As, Sb, and U) and result in their transport in fluids toward shallower reservoirs ( e.g., seafloor, forearc mantle wedge). The removal, by devolatilizatio n, and up-dip transport of such components toward the seafloor or into the forearc mantle wedge, could in part explain the imbalances betwee n the estimated amounts of subducted volatiles (e.g., H2O, B, N) and t he amounts returned via are magmatism. The results of this study demon strate that subduction can deliver, to various parts of the mantle (e. g., the forearc mantle wedge, the subarc mantle wedge, and deeper part s of the mantle), C-OH-S-N fluids, melts, and residual mineral reservo irs strongly fractionated isotopically and chemically relative to init ial compositions of the subducted rocks. Some compatibilities between the rock compositions and inferred fluid compositions for the Catalina Schist and the isotopic and trace element compositions of various man tle-derived materials (igneous rocks and xenoliths) indicate the excit ing potential to trace the large-scale transfer of these fractionated chemical signatures.